1. Field of the Invention
This invention relates to a semiconductor laser with improved high output power operation characteristics. More particularly, it relates to a semiconductor laser which is excellent in reliability at a high power output of 30 mW or more per facet and which can attain laser oscillation in a stabilized fundamental transverse mode up to such a high output power.
2. Description of the Prior Art
It is well known that stable operation of semiconductor lasers at a high output power is prevented by deterioration and/or damage to the light emitting-facets. The photon density in the vicinity of the facets is the highest, causing acceleration of the deterioration of the facets, so that absorption of laser light around the facets influences the life of semiconductor lasers to a great extent.
Window semiconductor lasers have been proposed which can be operated for a long period of time at a high output power. In these lasers facet-breakdown usually occurs when output power is increased, and the absorption of laser light around the facets is reduced to suppress deterioration of the facets. Examples of these layers are a window-stripe laser reported by Appln. Phys. Lett. 34(10), 15 May, 1979 p. 637 and a crank type TJS laser reported by Jpn. J. Appln. Phys. 21(1982) Supplement 21-1, p. 347. However, these window lasers do not have an optical waveguide in the direction parallel to the junction in the window region. Thus, laser light is propagated in the window region and the amount of reflected light from the facets to the laser operation area is reduced so that the oscillation efficiency is reduced, resulting in an increase in the oscillation threshold current level. FIG. 7 shows the propagation of laser light within a conventional window semiconductor laser, wherein window regions 2 and 2', respectively, are formed from both ends 5 and 5' (from which the laser light is propagated with a wave-front 6) of a striped laser operation area 1 to the facets 3 and 3' (from which laser beams are emitted).
Beam waists are positioned at the end portions 5 and 5' of the laser operation area 1 in the direction parallel to the junction, while they are positioned at the facets 3 and 3' in the direction vertical to the junction, resulting in an astigmatism. This astigmatism makes it difficult to concentrate light refracted by lenses, etc., in order to achieve optical coupling. In order to eliminate these drawbacks of the conventional window semiconductor laser, the applicant of this application for patent has filed Japanese Patent Application No. 57-91636 concerning a window semiconductor laser in which an optical waveguide is formed in the window regions. The features of this window laser are that the portion of the active layer in the vicinity of the facets is a plane shape and the portion of the active layer inside of the facets is a crescent shape, said crescent portion being formed into a buried heterostructure, and accordingly window regions are formed in the vicinity of the facets and a stabilized fundamental transverse mode can be attained up to a high output power. Due to the buried heterostructure of the crescent portion of the active layer, current which does not contribute to laser oscillation can be reduced so that laser oscillation in a high-order transverse mode can be suppressed. However, a part of the current injected into a channel on the substrate flows into the burying layers having a high resistance (namely, current leakage which does not contribute to laser oscillation arises), so that a necessarily high current level to achieve a high output power operation must be used and threshold current cannot be sufficiently reduced. Moreover, this type of laser lacks reliability in practical use.